feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

backend/internal/account/account.go

@@ -18,6 +18,7 @@ import (
 	"github.com/go-jet/jet/v2/qrm"
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/lib/pq"
 
 	"scrabble/backend/internal/postgres/jet/backend/model"
 	"scrabble/backend/internal/postgres/jet/backend/table"
@@ -57,8 +58,9 @@ type Account struct {
 	// VariantPreferences is the set of game variants (engine.Variant stable labels:
 	// "scrabble_en", "scrabble_ru", "erudit_ru") the player is willing to be matched
 	// into. It gates the New Game picker, the matchmaker and the friend-invite the
-	// player creates; an invited friend may still accept any variant. A new account
-	// defaults to Erudit only. Never empty — enforced on update and by a DB check.
+	// player creates; an invited friend may still accept any variant. A newly registered
+	// account defaults to DefaultVariantPreferences and a guest to
+	// GuestVariantPreferences. Never empty — enforced on update and by a DB check.
 	VariantPreferences []string
 	// IsGuest marks an ephemeral guest account: a durable row with no identity,
 	// excluded from statistics, friends and history.
@@ -562,9 +564,14 @@ func (s *Store) ProvisionGuest(ctx context.Context, browserTZ, language string)
 	if lang == "" {
 		lang = "en"
 	}
+	// A guest is narrower than a registered player: Эрудит alone, where the column default
+	// gives a registered account both Russian-alphabet games (DefaultVariantPreferences).
+	// The set is written explicitly for exactly that reason, and ClearGuest widens it to the
+	// default when the guest later registers.
 	stmt := table.Accounts.
-		INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.IsGuest, table.Accounts.TimeZone, table.Accounts.PreferredLanguage).
-		VALUES(accountID, guestDisplayName(), true, tz, lang).
+		INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.IsGuest, table.Accounts.TimeZone, table.Accounts.PreferredLanguage, table.Accounts.VariantPreferences).
+		VALUES(accountID, guestDisplayName(), true, tz, lang,
+			postgres.Raw("#guest_variants::text[]", map[string]interface{}{"#guest_variants": pq.StringArray(GuestVariantPreferences)})).
 		RETURNING(table.Accounts.AllColumns)
 
 	var row model.Accounts